The invention relates to a cathode-ray tube and, more particularly, to a cathode-ray tube having an electron gun with a cathode of a high current density in which stable electron emission characteristics are obtained for a long time.
Various cathode-ray tubes such as television picture tubes and image display electron tubes used as monitor tubes of information processing terminals have an electron gun for emitting one or a plurality of electron beams at one end of a vacuum envelope and a phosphor screen obtained by coating a fluorescent material film of one or a plurality of colors onto an inner surface at the other end of the vacuum envelope so that an electron beam emitted from the electron gun is two-dimensionally scanned by a magnetic field generated by a deflecting yoke attached to the outside of the vacuum envelope, thereby displaying a desired image.
For such a kind of recent cathode-ray tube, high definition of a display image is required in association with realization of a variety of display information, a high-density of display information. and the like. To realize high definition of a display image, it is necessary to remarkably improve focusing characteristics of the electron beam.
As means for improving the focusing characteristics to satisfy the requirement of the high definition mentioned above, it is considered to reduce an electron beam passage hole of a first grid electrode of the electron gun.
However, there arise a limitation on the reduction of the dimension of the diameter of the electron beam passage hole of the first grid electrode due to a limitation on a cut-off voltage, a limitation of an electrode manufacturing precision, and a limitation of a cathode loading. Particularly, the cathode loading relates to the life reliability of the cathode-ray tube and exerts a large influence in case of deciding the hole diameter of the first grid electrode.
Therefore, the reduction of the hole diameter of the first grid electrode is limited to no less than 0.40 mm except for a combined use of an impregnated cathode suitable for a high current density operation. The impregnated cathode, however, has problems such that the number of manufacturing steps is large and it is expensive.
A cathode-ray tube having a cathode with an electron emissive material layer to solve such problems has been disclosed in U.S. Pat. No. 5,216,320 issued Jun. 1, 1993, and assigned to the same assignee as that of the present invention. FIG. 1 is a cross sectional view for explaining a structure similar to that of a cathode shown in the above U.S. patent. Reference numeral 13 denotes a cylindrical cathode sleeve; 14 a cap-shaped metal base; 15 an electron emissive material layer; 16 a heater; 19 a first layer made of an oxide of alkaline earth metal; and 20 a second layer made of an oxide of alkaline earth metal containing a rare earth metal (for example, barium scandate Ba2Sc2O5).
The cap-shaped metal base 14, sealing one end of the cylindrical cathode sleeve 13, is made of a material made of refractory metal, for example, nickel (Ni) as a main component and containing therein a reduction metal of a small quantity of silicon (Si) or magnesium (Mg). The heater 16 is placed in the cathode sleeve 13, thereby constructing an indirectly heated cathode.
The electron emissive material layer 15 having a double layer structure is coated or formed on the upper surface of the metal base 14. The electron emissive material layer 15 includes the first layer 19 made of an oxide of alkaline earth metal coated or formed so as to be in contact with the upper surface of the metal base 14 and the second layer 20 made of an oxide of alkaline earth metal which is coated or formed on the surface of the first layer 19 and contains an oxide of rare earth metal such as barium scandate or the like.
The electron emissive material layer 15 with the above structure may be manufactured as follows. A first layer of a carbonate of alkaline earth metal is first formed on the upper surface of the metal base 14. A second layer of a carbonate of alkaline earth metal containing an oxide of rare earth metal such as barium scandate (Ba2SC2O5) or the like is formed on the first layer. After that, the carbonate of alkaline earth metal of each of the first and second layers is changed to an oxide of alkaline earth metal by a thermal decomposition at the time of a heat treatment in a manufacturing step of the cathode-ray tube, thereby obtaining the electron emissive material layer 15 comprising the first layer 19 and the second layer 20 laminated on the first layer 19.
According to the structure of the electron emissive material layer 15 as mentioned above, the second layer 20 made of the oxide of alkaline earth metal formed on the electron emissive surface side of the electron emissive material layer 15 and containing the oxide of rare earth metal such as barium scandate or the like binds free barium (Ba) formed by a reduction element in the metal substance 14 to the second layer 20 and maintains free barium in the electron emissive material layer 15 so as to be in a high density state.
Thus, even when the cathode operates in a high current density state, an amount of Joule heat generated in the electron emissive material layer 15 is reduced and a degree of evaporation of barium also decreases.
As mentioned above, even when the cathode shown in FIG. 1 is operated-in a high density current state, for example, in a large current state exceeding 2A/cm2, the reduction of current emission is small and the cathode of a long life can be realized.
In order to display an image on a display panel at high definition, it is necessary to reduce a diameter of an electron beam passage hole of an electrode, namely, a first grid electrode which is arranged adjacent to the cathode and has a hole to limit emanation of electrons emitted from the cathode.
According to the knowledge derived by experiments and study of the present inventors, an oxide of rare earth metal such as barium scandate or the like has functions of binding free barium and maintaining free barium in an electron emissive material layer in a high density state. However, since the rare earth metal oxide itself such as barium scandate or the like never contributes to the electron emission, when a content of the rare earth metal oxide such as barium scandate or the like in the electron emissive material layer is increased, an amount of emission of electrons from the electron emissive material layer decreases and the life of the cathode-ray tube is not always extended.
In addition, the rare earth metal oxide such as barium scandate or the like is an expensive material.
It is an object of the invention to provide a cathode-ray tube which is capable of displaying an image at high definition (high resolution) and has a sufficiently long operating life.
Another object of the invention is to provide an electron gun suitable for the above cathode-ray tube.
According to one aspect of the invention, there is provided a cathode-ray tube with an electron gun having a cathode, a control electrode, and another electrode, wherein the cathode has an electron emissive material layer for emanating electrons through a hole of the control electrode, the hole in the control electrode substantially having a diameter in a range from 0.3 mm to 0.4 mm, and the electron emissive material layer includes a first layer which is formed on a support and is made of an oxide of alkaline earth metal containing no oxide of rear earth metal and includes a second layer which is formed on the first layer and is mainly made of an oxide of alkaline earth metal substantially containing an oxide of rare earth metal of a quantity in a range from 0.8 wt % to 5.0 wt %.
As a diameter of electron beam passage hole formed on a first grid electrode (a control electrode arranged adjacent to the cathode) is smaller, a beam diameter at a crossover point in the electron gun more decreases and focusing characteristics are more improved. In case of a color cathode-ray tube, however, in addition to a cathode loading, a variation (dissidence) of cut-off voltages among three electron beams (R beam, G beam, B beam) occurs, and due to a limitation of manufacturing precision, there arises a limitation on the reduction of the hole diameter of the electron beam passage hole.
For adjustment of the cut-off voltages to constant values, with the diameter of the electron beam passage hole of the first grid electrode being reduced, a distance between the cathode and the first grid electrode has to be reduced, which will result in a larger cut-off voltage fluctuation due to a variation in dimensions between the cathode and the first grid electrode.
FIG. 2 is a characteristics graph showing a change in variation of cut-off voltages among three beams, for a specific cut-off voltage, in the case where the hole diameter of the first grid electrode is reduced. An axis of ordinate indicates a ratio of the highest cut-off voltage to the lowest cut-off voltage.
According to the current electrode manufacturing technique, as shown in the diagram, when the diameter of the electron beam passage hole of the first grid electrode is less than 0.3 mm, the variation of the cut-off voltage is large and is intolerable for a practical use.
The effect of dispersion of the rare earth metal oxide, such as barium scandate or the like in the alkaline earth metal oxide of the cathode will be, unless its dispersion density exceeds a certain value, such that durability of the cathode against the cathode loading increases in accordance with an increase of the dispersion density, so that under the same condition, the life time (a time period in which a value in percentage of a ratio of an initial maximum anode current to a maximum anode current after a lapse of a time becomes 50%) is extended.
FIG. 3 is a characteristics diagram showing a transition of a ratio with respect to an initial maximum anode current for a diameter d of the electron beam passage hole of the first grid electrode being 0.40 mm in which cathode-ray tubes having electron guns using cathodes in which a dispersion amount of an oxide of rare earth metal such as barium scandate or the like in the oxide of alkaline earth metal is changed is operated under the same test conditions.
In the diagram, curves 22, 23, and 24 show cases where cathodes have dispersion densities equal to 0.8 wt %, 1.6 wt % and 3.0 wt %, respectively.
It will be also understood from this characteristics diagram that when the dispersion density of the rare earth metal oxide such as barium scandate or the like in the alkaline earth metal oxide is increased, a reduction in the ratio of a maximum anode current with respect to the initial maximum anode current becomes gentle and the durability against the cathode loading increases and the life time as a cathode-ray tube becomes long.
However, if it is intended to improve the focusing characteristics by further reducing the hole diameter of the electron beam passage hole of the first grid electrode in order to satisfy the recent requirement for realization of high definition of a display image. It may be considered better to further increase the dispersion amount of the rare earth metal oxide such as barium scandate or the like.
However, as mentioned above, since the rare earth metal oxide itself such as barium scandate or the like never contributes to the emission of electrons, the increase of dispersion amount contrarily results in a decrease in the amount of emission of electrons from the electron emissive material layer.
FIG. 4A is an explanatory diagram showing a change in initial electron emission capability of a cathode-ray tube having an electron gun using a cathode in which a dispersion amount of barium scandate in the alkaline earth metal oxide is changed. The initial electron emission capability is shown by normalizing it by an initial electron emission capability at 0 wt % dispersion amount. The diameter d of the electron beam passage hole of the first grid electrode of the electron gun is equal to 0.4 min.
FIG. 4B is an explanatory diagram of a life time of a cathode-ray tube having an electron gun using a cathode in which a dispersion amount of the rare earth metal oxide such as barium scandate or the like in the alkaline earth metal oxide is changed. The diameter d of the electron beam passage hole of the first grid electrode of the electron gun is equal to 0.35 mm.
As will be understood from FIGS. 4A and 4B, when the dispersion amount of barium scandate in the alkaline earth metal oxide is equal to or larger than 5%, the electron emission amount obviously decreases (FIG. 4A), and further, when the dispersion amount approaches the same dispersion amount 5% where the electron emission capability decreases to 90 to 95% as compared with the initial electron emission capability with the dispersion amount of 0 wt %, even if the dispersion amount is increased any more, the life time of the cathode-ray tube is saturated or decreases (FIG. 4B).
It has been found that the above tendency regarding FIGS. 4A and 4B occurs independent of the value of the diameter of the electron beam passage hole of the first grid electrode of the electron gun.
As mentioned above, when the content of the rare earth metal oxide such as barium scandate or the like is increased in order to improve the high density current operating characteristics, the electron emission amount of the cathode decreases and, further, the life as a cathode-ray tube becomes short. It is, therefore, necessary to decide the dispersion amount of the rare earth metal oxide in accordance with the using conditions (typically, the diameter of the electron beam passage hole of the first grid of the electron gun).
FIG. 5 is a characteristics diagram in which the relation between the cathode loading and the life time was obtained with respect to five kinds of cathodes in which the dispersion density of the rare earth metal oxide such as barium scandate or the like was changed. Reference characters A, B, C, D, and E show cases using cathodes in which the dispersion amounts of barium scandate are equal to 0 wt %, 0.8 wt %, 1 wt %, 3 wt % and 5 wt %, respectively.
The diagram shows that when the diameter of the electron beam passage hole of the first grid electrode is reduced for the same cathode current, the cathode loading (A/cm2) becomes heavier and the life becomes shorter accordingly.
With the conventional cathode containing no rare earth metal oxide, when the diameter of the electron beam passage hole of the first grid electrode is lower than 0.40 mm (in the diagram, shown as xcfx860.40, and the same shall also apply hereinbelow), the life time of the cathode will be so short that a cathode-ray tube with such conventional cathode is supposed to be unacceptable in the market and the cathode will need improvement. That is, considering the life characteristics of the cathode-ray tube, in the conventional oxide cathode containing no rare earth metal oxide, except for the combined use with the impregnated cathode suitable for the high current density operation, a limit value of the reduction of the diameter of the electron beam passage hole of the first grid electrode was 0.40 mm.
On the other hand, in order to satisfy the recent requirement of the high definition display image, as means for improving the focusing characteristics, it is necessary to set the hole diameter of the electron beam passage hole of the first grid electrode to a value smaller than 0.40 mm. As will be also understood from FIG. 2, however, in case of considering the current electrode manufacturing precision and the limitation of a variation in spot extinguishing voltage (cut-off voltage) of each cathode, the limit value of the reduction of the hole diameter of the electron beam passage hole of the first grid electrode is 0.30 mm. When the variation of the cut-off voltage increases to a value near 1.3, a load to a circuit to adjust each cathode voltage of the cathode-ray tube in the television or display monitor increases and it is not practical in the present market from a viewpoint of the costs.
Therefore, even if the diameter of the electron beam passage hole of the first grid electrode is less than 0.40 mm, by increasing the dispersion amount at least up to 0.8 wt % to a line B in FIG. 5) by allowing the rare earth metal oxide such as barium scandate or the like to be contained, the effect of improving the life characteristics can be sufficiently obtained. Moreover, when the dispersion amount is equal to 0.8 wt %, even if the diameter of the electron beam passage hole is equal to 0.30 mm, life characteristics which are almost equivalent to those in case of combining the conventional cathode containing no rare earth metal oxide and the first grid electrode in which the diameter of the electron beam passage hole is equal to 0.40 mm are obtained. Further, good life characteristics can be maximally effected at the dispersion density of 3.0 wt % (refer to a line D in FIG. 5) in a range where-there is no change in initial electron emitting characteristics.
More preferably, when the diameter of the electron beam passage hole is equal to 0.30 mm, in order to obtain a life time that is almost equal to that in case of combining the conventional cathode containing no rare earth metal oxide and the first grid electrode in which the diameter of the electron beam passage hole is equal to 0.40 mm, the dispersion amount should be increased to about 1.0 wt %. When the dispersion amount is equal to 0.8 wt %, in order to obtain a life time that is almost equal to that in case of combining the conventional cathode containing no rare earth metal oxide and the first grid electrode in which the diameter of the electron beam passage hole is equal to 0.40 mm, the diameter of the electron beam passage hole should be about 0.33 mm.
In the above dispersion, as an oxide of rare earth metal to be contained in the oxide of alkaline earth metal serving as an electron emissive material layer, use of a europium oxide (Eu2O3), a scandium oxide (Sc2O3), or an yttrium oxide (Y2O3) in place of barium scandate (Ba2Sc2O5, BaSc2O4, or Ba3ScO9) offers a similar effect.
As mentioned above, by selecting the dispersion amount of the rare earth metal oxide such as barium scandate or the like in the alkaline earth metal oxide in accordance with the using conditions of the cathode, the electron gun having excellent high current density operating characteristics and satisfactory electron emitting characteristics can be obtained at relatively low cost.